File photo dated 01/10/13 of Work and Pensions Secretary Iain Duncan Smith, who will declare that work is &quot;good for your health&quot; and can help people recover from illness as he unveils plans for more reforms in the welfare system.

Iain Duncan Smith has said that imposing a four-year ban on migrant benefits is a ‘crucial’ test for David Cameron’s EU reforms.

The Work and Pensions Secretary insisted that the Government intended to deliver on its Tory manifesto pledge to bar EU nationals from claiming in-work benefits such as tax credits.

Mr Duncan Smith refused to rule out imposing a similar ban on British youngsters if Brussels blocked the migrant plan, but stressed that the main focus was on making sure Tory promises to end ‘benefit tourism’ became a reality.

And just days after Mr Cameron admitted that he did not expect to get agreement on his package of reforms this month, Mr Duncan Smith claimed that the delay was a sign of ‘strength’ not weakness.

The Tory manifesto in May stated explicitly that the Government “will insist that EU migrants who want to claim tax credits and child benefit must live here and contribute to our country for a minimum of four years”.

But on BBC1’s Andrew Marr Show on Sunday, Mr Duncan Smith made clear that he expected the PM not to back down.

Asked if the four-year plan was an ‘acid test’ for the EU renegotiation, Mr Duncan Smith replied: “There’s a lot of things in this negotiation, so it’s not just one thing. I know that the Prime Minister has made it crucial to himself in the sense that he’s always talked about you know what we have to deliver.

“[On Monday] The Prime Minister’s making a speech reminding the Conservatives that we were elected on a manifesto and we need to deliver on that manifesto regardless of the state of the opposition. And of course the manifesto commitments were very clear on this.”

He added: “I have complete faith in the Prime Minister wanting to deliver on what he said he would

at the time of the election.”

If other states block the four-ban as illegal or unworkable, some in No.10 have floated the idea of a four-year ban on all nationals - including young Britons - as one way round the problem.

The 'contributory principle' idea, suggested by Downing Street adviser Max Chambers, would mean that Britons who left university wouldn't be eligible for benefits until they reach 25, while those who start work at 18 couldn't be eligible until they reach 22.

Asked today about such a fall-back plan, Mr Duncan Smith said: This has never been part of our proposal.”. And pressed further on whether he’d opposed it, he replied: “I’m not going to put a position down on this in the sense that all.”

“I’m in the business with the prime minister of delivering what the Prime Minister said he wanted at the time of the election through the manifesto, and what he has said clearly to his European colleagues is we need to do this.

“The delay, which everyone said oh it’s a sign of weakness, I think it’s a sign of strength. It says the Prime Minister’s determined not just to come back with any deal, but to come back with a deal that he knows he can sell to the British people.”

A Vice-pesident of the Vote Leave campaign, he said the renegotiation process was "very lame" and "relatively trivial". "OK, he gets the deal by February - it won't change the price of fish," he told BBC Sunday Politics.